1. Field of the Invention
The present invention relates to a fabrication method for integrated circuits. More particularly, the present invention relates to a method of fabricating a semiconductor device.
2. Description of the Related Art
Progress in semiconductor fabrication technologies has made it possible to fabricate semiconductor devices at the deep sub-micron level. As the sizes of devices decreases, it is necessary to control effectively a junction depth and a channel depth, in order to obtain a decreased threshold voltage and to prevent a short channel effect.
Of the many currently used P-type dopants, boron atoms (B) are most widely used. However, the boron atoms have a high diffusion coefficient. Thus, it is difficult to form a high-quality shallow junction by implanting the boron atoms. In order to control the junction depth and the channel depth, dopants having properties such as a high atomic mass, a low diffusion coefficient and a sufficient solubility in silicon are implanted. In the group-III elements, indium (In) atoms have a high atomic mass and a low diffusion coefficient, which is about 5 to 10 times lower than that of the boron atoms. Hence, implanting indium atoms as the P-type dopant has become popular.
However, indium atoms have a high energy gap, which is usually above 0.16 KeV. Additionally, the radius of an indium atom is larger than the radius of a silicon atom. Therefore, the indium atoms easily gather to form clusters due to the tendency to reduce the stress between the indium atoms and silicon substrate. If the indium atoms form clusters, the indium atoms cannot effectively serve as acceptors. Thus, the effective concentration of the implanted indium atoms cannot be increased as the implantation dosage increases during ion implantation. Actually, the effective concentration of the implanted indium atoms gradually approaches a concentration saturation point. The foregoing effect is called a freeze-out effect, which can be observed after performing ion implantation and annealing. Reference is made to FIG. 1, which shows the freeze-out effect observed by a graph showing the relationship between a dosage and a sheet conductance illustrated after performing ion implantation and annealing. The horizontal axis represents dosage, measured in 1/cm.sup.2. The vertical axis is sheet conductance, measured as a square unit/K ohm.